1. Field of the Invention
The present invention relates to a capacitance sensor apparatus of divided multi-electrode type having a plurality of sensor circuits continuously arranged for detecting a change in external electrostatic capacitance.
2. Description of the Prior Art
Apparatuses for detecting paper thickness are known. One apparatus uses a potentiometer and another apparatus is designed to detect a change in thickness as a change in electrostatic capacitance. This change in electrostatic capacitance can be measured as a change oscillating frequency of an oscillator circuit in accordance with the change in electrostatic capacitance. However, these apparatuses have poor detection precision and cannot detect the thickness and surface state of a sheet-like medium, e.g., paper or a film with a high sensitivity and high precision.
The present applicant previously proposed a capacitance sensor apparatus which can detect a change in small electrostatic capacitance with a high sensitivity of 1.times.10.sup.-5 pF in U.S. Pat. No. 5,231,359, issued Jul. 27, 1993. This capacitance sensor apparatus will be referred to as '359 patent. As shown in FIG. 1, the '359 Patent is constituted by: a sensor circuit comprising an oscillator circuit 14 having a ceramic resonator 19, a resonance circuit 21 having a ceramic resonator 15 different from the ceramic resonator 19 of the oscillator circuit 14, a detector circuit 16, an amplifier circuit 17, and an AFC (Automatic Frequency Control) circuit 18; and a detector electrode 20.
The oscillator circuit 14 outputs an oscillation frequency signal having a fixed high frequency of 0.5 to 10 GHz. The detector electrode 20 confronts an object to be detected to detect a change in small electrostatic capacitance with respect to the object to be detected and includes the detected change in small electrostatic capacitance to the resonance circuit 21. The resonance circuit 21 changes the resonance frequency in accordance with the change in small electrostatic capacitance detected from the detector electrode 20, forms an amplitude modulation signal corresponding to the change in resonance frequency by using the oscillation frequency signal from the oscillator circuit 14 as a carrier, and applies it to the detector circuit 16 as the detection signal. The detector circuit 16 envelope detects a detection signal, converts it to a signal within the detection band of the small electrostatic capacitance detected by the detector electrode 20, and applies this signal to the amplifier circuit 17. The amplifier circuit 17 applies the envelope detection signal detected by the detector circuit 16 to a desired signal processing circuit. The AFC circuit 18 corrects the resonance frequency of the resonance circuit which is biased by such influence as an environmental change to stabilize the resonance frequency.
According to the '359 Patent, since the change in small electrostatic capacitance detected by the detector electrode 20 is detected as a change in resonance frequency of the resonance circuit 21, a change in small electrostatic capacitance which is as small as 1.times.10.sup.-5 pF can be detected with a high sensitivity.
The present inventor derives from the '359 Patent and develops an apparatus suitable for detection of a sheet-like medium by utilizing this prior art.
To apply the prior art to detection of a sheet-like medium, a detector electrode 20 is constituted by an electrode plate having a length almost equal to the width of a sheet-like medium 3, as shown in FIG. 3. For example, as shown in FIG. 2, a ground electrode P is disposed to oppose the detector electrode 20 at a gap S through which the sheet-like medium 3 passes. A change in electrostatic capacitance, when the sheet-like medium 3 passes between the detector electrode 20 and the ground electrode P, it is detected by the detector electrode 20. The detected change is applied to the ceramic resonator 15 of the circuit shown in FIG. 1, thereby detecting the thickness, humidity-absorbing state, and surface roughness of the sheet-like medium 3, or a foreign material attached to the sheet-like medium 3.
However, when the detector electrode 20 is set to have a length almost equal to the width of the sheet-like medium 3, as shown in FIG. 3, the surface area of the detector electrode 20 is increased to increase its fixed capacitance Co. The resolution is decreased when the small electrostatic capacitance is to be detected.
In order to eliminate this problem, as shown in FIG. 4, a plurality of small detector electrodes 20 may be arranged in the widthwise direction of a sheet-like medium 3 to be connected to sensor circuits 22 similar to that shown in FIG. 1. When the plurality of capacitance sensor circuits 22 are arranged adjacent to each other in this manner, and the oscillating frequencies of oscillator circuits 14 of the respective capacitance sensor circuits 22 are slightly different from each other, the oscillating frequency of adjacent circuits can interfere with each other. This degrades the detection capability of the small electrostatic capacitance. Furthermore, when the sensor circuits are connected respectively to the plurality of detector electrodes 20, the number of capacitance sensor circuits 22 is largely increased to complicate the overall configuration, resulting in an increase in apparatus cost.